The Cowbird’s Guide to Practical Brood Parasitism

So I’m willing to bet that a lot of you are aware of brood parasitism à la the cuckoo, and a good number of my followers have probably even heard of the terrifying methods the intraspecific brood parasitic coot uses to weed out the fakers from its progeny.

So let’s talk about brood parasitism and why it’s good and why it’s not so good and the different strategies that different bird species use, including mafia behavior. And we’ll talk about the development of male cowbird courtship too because that’s kind of cool. But yeah, lots of bird child murdering coming up just so you’re aware.

The brown-headed cowbird (Molothrus ater) has a range across North and Central America, depending on the time of year. It is in the same family as blackbirds and grackles. Here’s a male and female pair on a roof.

(Photo by Lisette Lebailiff.)

The male is the dark-colored one with a brown head from which the species gets its name.

Now, if you are familiar with birds and with sexual selection, you could postulate that since the species is sexually dimorphic (the male and female look different) they probably aren’t socially monogamous. And you’d be right. While there is some mate-guarding behavior in both male and female cowbirds, both sexes are known to mate with multiple partners.

The brown-headed cowbird isn’t monogamous because it doesn’t need to be monogamous. Social monogamy in birds has an important function- raising altricial offspring is often a lot of work, and two parents pooling their resources together generally means more offspring survive to adulthood.

(Note that I’m specifying social monogamy, not sexual monogamy- birds cheat on each other all the freaking time.)

The brown-headed cowbird needs no monogamy in any form because it does not raise any of its own offspring. It is an obligate parasitic brooding species- meaning, not only does it rely on others to raise its young, but it literally no longer has the ability to raise them itself.

To describe brood parasitism, I’ll go back to the most famous brood parasite of them all- the cuckoo, though it’s a bit of a misnomer, because the cuckoos are actually a large family (Cuculidae) in which most species are actually not brood parasites.

Of the species that do practice brood parasitism, however, the common cuckoo is the example we’re all familiar with. Like the brown-headed cowbird, it is an obligate brood parasite and never raises its own offspring. It sounds like an easy, lazy strategy, but is in fact quite demanding given that the host birds would rather not raise the cuckoo’s offspring, thanks.

The common cuckoo’s strategy to get its hosts to raise its offspring is namely trickery: that is, it tricks its hosts into thinking its eggs are their own. In order to do this, it first has to get its intended host parents away from their nest. It does this either by waiting until the nest is unoccupied, or tricking the host parents into flying away. There are at least two lines of common cuckoo females imitating different predatory raptors. This is an example of polymorphism, or different physical appearances occurring within the same species.

Different color morphs of the common cuckoo, likely imitating the sparrowhawk and kestrel.

This mimicry can get difficult, as the host parents tend to learn the difference between mimics and actual predators over time. This explains the reasons that TWO morphs have evolved- as the hosts in one area get more familiar with one, the less popular morph makes a comeback and they have to learn all over again.

Once the host parents are away, the cuckoo gets to work. Generally it lays one egg in each host nest, with a single cuckoo capable of laying 50 or more eggs each season. (It also tosses out one of the hosts’ eggs, because birds can count.) Individual cuckoo females preferentially go after specific species, and have actually evolved many different egg colorations to match those species.

Cuckoo eggs (the largest in each clutch) mimicking reed warbler eggs. (Photo by Chiswick Chap.)

The genetic lines of common cuckoos that lay differently-colored eggs are called ‘gentes.’ It’s a second example of polymorphism within common cuckoos. I have no idea if there’s research showing whether or not different gentes are also associated with different female feather color morphs (if you know of any, I’d love to see it!)

There’s been a flurry of research trying to determine how these different egg-colorations are passed down from cuckoo to cuckoo. One argument seems to be that the egg coloration genes are passed down through matrilines, from mothers to their daughters, and that the males have little to nothing to do with it. This means that common cuckoo males can mate with any females without messing with those genes, preventing gentes of the common cuckoo from undergoing sympatric speciation and becoming separate species.

However, there’s been some recent evidence that the males may in fact influence the gentes in some way. It’s possible that eventually we could see speciation of different common cuckoo gentes targeting specific host species.

This appears to have already happened in another group of parasitic birds, the Viduidae family (indigobirds and whydahs). Each species in Viduidae parasitizes a different finch species, mimicking everything from their egg coloration down to the specific gape pattern that their nestlings have (i.e., that wide-open mouth that many baby birds have).

Pin-tailed whydah chick (larger) mimicking the gape pattern of a common waxbill. (Photo by Justin Schuetz.)

The gape patterns are pretty complex, likely having evolved in response to brood parasitism in the first place, and gentes of Viduidae likely followed until they speciated into the 19 specialized species of indigobird and whydah that exist today.

Anyway, back to the cuckoo’s parasitism. If the cuckoo is successful in tricking the host parents, its egg will hatch earlier than the host offspring, and the nestling will proceed to shove all of its adoptive siblings out of the nest like a murderous naked pterodactyl.

Then the cuckoo stops bothering to mimic the looks of the host’s nestling, because it’s already won. The host parents feed their horrible engorged changeling until it fledges. The baby cuckoo has to compensate for the fact that it needs as much food as an entire brood but has only one mouth (and the parents would therefore only feed it as much as they would one of their own babies). It actually has a specialized cry that sounds like a whole brood begging to stimulate the parents to bring it more food.

(Photo by Per Harold Olsen.)

It’s kind of obvious why the host birds don’t like this. It’s a big old loss because they raise no children that year, AND they exhaust themselves trying to raise this god damn squatter. Unfortunately, once the nestling hatches, they’re pretty much hardwired to raise it. You don’t want to evolve offspring discrimination and then risk not raising one of your own kids, right? After all that energy investment taking care of the eggs, that’d be pretty bad. Then YOU’RE the asshole.

It’s better to detect the eggs right at the start and get rid of them. So, the host birds are actually locked in an evolutionary arms race with their cuckoo parasites, in terms of egg coloration. Hosts appear to be constantly evolving to change how their eggs look, with cuckoos evolving to catch up and keep mimicking.

That is how the common cuckoo does things- deception and subtle assassination. And a great deal of evolutionary complexity that is probably still increasing.

But the brown-headed cowbird does things a bit differently. Here is a picture of a nest parasitized by a brown-headed cowbird. Can you tell which one is the cowbird egg?

This is not a trick question. It’s obviously the speckled bluish one. Brown-headed cowbirds don’t bother trying to match the eggs of their host.

This seems like a problem right off the bat, but think about it. Common cuckoos more or less depend on being able to match the eggs of their host, which means each individual only has one or two they can focus on at a time. A single lady cowbird, on the other hand, can parasitize as many different species as she wants to.

The question, of course, is how do the cowbirds get the host birds to accept their very different-looking eggs?

Well, to start with, getting parasitized by a cowbird isn’t QUITE as bad as being parasitized by a cuckoo. Unlike cuckoo nestlings, cowbird nestlings will usually leave their siblings well enough alone. I mean, there are reports that larger cowbird nestlings will occasionally smother their siblings, yes, but that might be accidental.

The size difference between cowbirds and their hosts can be… er… impressive. In cases like this, small birds can lose all their young simply because the larger cowbird nestling is super demanding and takes all the food and energy the parents have. Interestingly, there’s evidence that some host nestlings respond to the presence of parasitic nestlings by begging more loudly to match it. Of course, this means that the entire nest is more likely to be spotted by predators.

But in general, a cowbird is less deadly to its hosts than a common cuckoo is. And this might be a large part of why cowbird hosts tolerate their eggs. I mentioned earlier that it’s not a great idea to evolve a behavior to throw out babies that don’t look like yours, because this increases the chance of accidentally throwing out your own babies (the exceptions to this can be found in intraspecific brood parasites, i.e. species that parasitize their own kind, because it’s kind of a necessity. Refer back to the coots).

In a case where at least some of your offspring have a chance of surviving parasitism, it might be less costly to accept the parasite than it would be to risk throwing out some of your own kids.

Ok, so the brown-headed cowbird is slightly nicer to its hosts than the common cuckoo is… right?

Ok, I already spoiled it at the beginning, so I’ll go ahead and say it. The brown-headed cowbird may have another method of ensuring that its host parents have… no choice but to accept their offer.

Some hosts still do reject cowbird eggs, regardless of apparent evolutionary cost, either by abandoning the nest, building a new nest on top of the parasitized brood (harsh), or by simply removing the egg itself. In the first two cases, it means the parents have to completely start their brood over, so it’s a big cost. That’s why these two strategies usually occur in birds that stand to lose their entire brood if the cowbird hatches. Cutting their losses, so to speak.

However, female cowbirds may do more than just lay their eggs and jet. There’s evidence that female cowbirds return to the nests that they parasitize, to… check in. And if they happen to notice that their little baby is missing, well…

They throw the hosts’ eggs or offspring out of the nest. All of them. So that they die.

This “mafia behavior” (actual scientific term) has double benefits. First, it makes rejecting cowbird offspring much more costly than accepting them. Second, if it’s the correct time in the season, the host parents will lay another brood, which Mama cowbird can then reparasitize. This is termed “farming,” and can also occur in nests that have never been parasitized to make the host’s brooding match up with the cowbird’s.

(Side note: The description of one way female cowbirds select their preferred host nest in one paper was “cryptic, silent watching of nest-building hosts,” and that’s just a little terrifying.)

We don’t know how often this behavior actually occurs in brown-headed cowbirds, as there are only scattered observations and one experimental paper on the subject, and that paper only examined one population of cowbirds and one host species (the prothonotary warbler). But for this single case, the evidence was compelling. When researchers removed cowbird eggs from warbler nests, the nests would be destroyed 56% of the time, compared to 6% of the time if no eggs were removed. If cowbird access to the nests was restricted, no nests were destroyed at all.

Evidence of farming was also found- 20% of nests never initially parasitized were destroyed. Of all the nests that were destroyed- parasitized or not- 85% of them were reparasitized when the host parents laid more eggs.

If mafia behavior is common, then it would actually be in the host birds’ favor to evolve to become LESS discriminatory, because it ensures that at least some of their offspring will survive versus none. And thus the hosts hunker down and care for the intruders under the female cowbirds’ watchful gaze.

Fuckin’ birds, am I right?

So that’s the visceral terror of having your nest parasitized by a brown-headed cowbird.

I could close on that, but there’s one more really interesting thing about the brown-headed cowbird I wanna talk about that basically shows just how bizarre brood parasitism can make things.

So, male brown-headed cowbirds.

We haven’t given the males any attention for a while, poor things. (Photo by Bear Golden.)

These guys all grow up with parents that aren’t members of their species. This becomes rather important when it comes to things like… song learning. Unlike the common cuckoo, the brown-headed cowbird doesn’t instinctively know the proper song. It has to learn its courtship calls. But there’s a problem. The cowbird’s host parents aren’t cowbirds. How are they going to teach him cowbird courtship calls?

Well, they don’t, obviously, because the female cowbird hasn’t figured out a hostage situation to solve that little issue yet. But this presents a real problem for male cowbirds because… you gotta impress the girl if you’re gonna mate. And- lady-lovers take note- you CAN’T IMPRESS A GIRL IF YOU DON’T HAVE A GOOD MATING CALL.

I know this from experience. (Photo by Jan Malik.)

So how do male cowbirds ever get a chance to mate? Where do they learn their songs?

Well, once juvenile cowbirds learn how to fly, they say goodbye forever to their host parents and join flocks of other juvenile and adult cowbirds. At this point, they don’t even really have songs, just crappy imitations of whatever they learned from their host parents.

(There’s been no research that’s successfully figured out how cowbirds learn to recognize their own species, by the way.)

But once they join flocks of their own species, juvenile cowbird males learn songs by listening to adult males and by observing the reactions of adult females. It can take them up to two years to perfect their cowbird-sounding songs, and it takes them about that long to actually get females interested in them.

There’s actually probably a good reason that females wait until male songs are perfected: it shows them the males’ age and therefore potential quality as a mate. Yearling males have less experience than males two years and older, which means that they’ve had fewer chances to act stupid and die. No female wants to accidentally accrue “act stupid and die” genes for her offspring if she can help it, and there’s the added bonus that good song learning is an important indicator of good mental ability.

The female cowbird is an integral part of how the male learns to sing, even though females do not sing themselves. This becomes evident in studies examining populations of cowbirds with different dialects- like many species, cowbird songs are slightly different based on their region.

Juvenile males placed with females and males from their region learn their normal dialect, of course, and juvenile males placed with females and males from a different region learn THAT dialect. However, when juvenile males are placed with males from one region and females from another, they learn the song that the females like and not the ones that the males sing- even though, again, the females never sing themselves!

The method that the females use to “teach” the young males their preferred songs is essentially positive reinforcement. The more the females like, the more they stick around to listen- and, eventually, the more they display come-and-ravish-me behaviors. It’s obvious why the young males would pay more attention to this feedback than the songs of their resident adult males.

I mentioned the Viduidae family up there earlier when talking about a brood parasite that has mimicked their hosts to the extent that they have actually speciated. Well, that goes for the songs too. Indigobirds and whydahs actually learn their songs from their host parents and use them to woo their same-species mates, another reason for speciation to occur- Viduidae females are only interested in males that sing the songs they themselves were raised hearing. So, females will only mate with males raised by the same host species.

There’s a hell of a lot more I could talk about on the subject (there’s a hell of a lot of research out there on cowbirds, and cuckoos too) but I hope you lot are satisfied by this relatively brief overview. I’ll put a lot of full text papers in the reference section if you want to read further.

Perhaps I’ll make another post on brood parasitism in non-bird species (it can happen occasionally in insects and fish as well) but for now, take this away from the cuckoos and the cowbirds: brood parasitism is NOT as easy as it sounds. It’s far more than just letting someone else do the work of raising your kids. Rather, the species that become obligate brood parasites and lose their own nurturing behaviors may be caught in a bit of a deadlock, essentially subject to the whim of their host species. There’s a reason cuckoos and cowbirds lay 40-50 eggs in a season: a lot of them fail. A heck of a lot.

Time may be the biggest factor in increasing complexity in the strategies and specialization of brood parasites. There’s evidence to suggest that common cuckoos have been brood parasites for a lot longer than brown-headed cowbirds have- about 60 million years based on studies of ancient taxa, versus less than 3 million years for the cowbird. This is plenty of time for the cuckoos to be forced to become more and more specialized as their hosts become more and more wise to their tricks.

The tendency both cowbirds and cuckoos have of murdering the young of their host species may seem a little horrifying, but what’s more horrifying to the parasitic birds is the fact that if their hosts stop accepting their eggs, their own existence will be effectively extinguished.